1. Field of the Invention
The present invention relates to a rechargeable battery parameter estimation apparatus and a rechargeable battery parameter estimation method for estimating an offset error of measurements of a current flowing through a rechargeable battery, and estimating a capacity error, which is a difference of a typical full charge capacity (FCC) of the battery measured in advance from an actual FCC of the battery at a time when the battery is in use.
2. Description of the Related Art
In order to efficiently use a rechargeable battery such as a lithium-ion battery, a nickel metal-hydride battery, and a lead-acid battery, it is indispensable to highly precisely estimate a state of charge (SoC) of the battery. However, the SoC of the battery is not a physical quantity that is directly measurable, and hence the SoC has to be estimated from a current flowing through the battery, a voltage between terminals of the battery, measurements of the battery temperature, and histories of these physical quantities. As the most basic methods for estimating the SoC of the battery, the open circuit voltage method and the Coulomb-counting method are known.
The open circuit voltage method is a method in which a rechargeable battery is leaved out offline, and the voltage between terminals of the battery which sufficiently reaches the thermodynamic equilibrium, namely, an open circuit voltage (OCV), is measured, then the SoC of the battery is calculated on the basis of a dependence of the OCV on the SoC, which is determined in advance. Therefore, the battery has to be suspended for a long period during the SoC estimation by means of the open circuit voltage method.
On the other hand, the Coulomb-counting method is a method in which the current flowing through the battery is integrated, and the integrated current is divided by an FCC of the battery, to thereby calculate a change in the SoC from the integration start time. The Coulomb-counting method is capable of estimating the SoC even while the battery is online, by considering a state in which the SoC is apparent, that is, a state in which the battery is fully charged or fully discharged as a reference. However, the Coulomb-counting method has such a problem that an estimation accuracy of the SoC is greatly degraded by a small off set error of the current measurements, which is inevitable in widely used Hall effect sensors.
As a related-art apparatus using the Coulomb-counting method, there is disclosed a battery charging rate estimation apparatus, in which a rechargeable battery is assumed as a linear system based on an equivalent circuit model of the battery, the current flowing through the battery is assumed as an input of the linear system, and the voltage between the terminals of the battery is assumed as an output of the linear system; a Kalman filter is then constructed on the basis of an extended system in which the offset error of the current measurements is appended to the state of the linear system, to thereby precisely estimate the SoC of the battery and the offset error of the current measurements (for example, refer to Japanese Patent Application Laid-open No. 2012-57964).
As a related-art apparatus combining the open circuit voltage method and the Coulomb-counting method, there is disclosed a battery capacity calculation apparatus in which a current flowing through a rechargeable battery is integrated only in a period in which an absolute value of the current flowing through the battery is equal to or more than a certain value and the influence of the offset error of current measurements is thus relatively small, and comparing changes in the SoC with that in the integrated current in this period, to thereby estimate the FCC of the battery (for example, refer to Japanese Patent Application Laid-open No. 2012-58028).
According to Japanese Patent Application Laid-open No. 2012-58028, the SoC is estimated from the OCV, and the OCV is acquired by means of recursive calculation based on an equivalent circuit of the battery. Characteristic parameters of the equivalent circuit are adjusted by an adaptive mechanism from transitions of the current flowing through the battery and the voltage between terminals of the battery. As a result, the influence of the offset error of the current measurements is affirmed to be decreased by limiting the current integration period to the period in which the absolute value of the current is equal to or more than the certain value.
However, the related arts have the following problems.
According to Japanese Patent Application Laid-open No. 2012-57964, the typical FCC of the battery and the characteristic parameters of the equivalent circuit are considered to be known, and hence there is such a problem that when the typical FCC of the battery and the characteristic parameters of the equivalent circuit have errors, those errors propagate to the estimated value of the SoC and that of the offset error of the current measurements.
On the other hand, according to Japanese Patent Application Laid-open No. 2012-58028, the current flowing through the battery, which includes an offset error, is referred to by the adaptive mechanism for adjusting the characteristic parameters of the equivalent circuit. The characteristic parameters of the equivalent circuit thus has an error caused by the offset error. This error propagates to the estimated value of the OCV, which is calculated in a manner that depends on the characteristic parameters of the equivalent circuit, further to the estimated value of the SoC, and finally to the estimated value of the FCC of the battery.
Further, for example, even if the FCC of the battery is estimated by using the technology disclosed in Japanese Patent Application Laid-open No. 2012-58028, and the offset error of the current measurements is estimated by using the estimated value of the FCC by means of the method disclosed in Japanese Patent Application Laid-open No. 2012-57964, the estimated value of the offset error is superimposed with an error caused by the actual offset error of the current measurements, and hence the correct value of the offset error cannot be obtained.
As described above, in the related-art apparatuses, there is such a problem that the FCC of a rechargeable battery cannot be correctly estimated as long as the measurements of the current flowing through the battery have an offset error, because the error caused by the offset error propagates the estimated value of the FCC of the battery.